2022 University of Minnesota Cooperative Black Cutworm Trapping Network
Report #6 May 14- 20, 2022
For more information: https://z.umn.edu/bcw-reporting
May 26, 2022
Greetings:
Small numbers of black cutworm moths continue to move into MN, only a trap
in Olmsted County detected a significant moth capture (Figure 1, Table 1). This follows a storm system into SE Minnesota and other nearby counties with the same weather might have seen similar immigrations. Depending on tillage planting progress in that area some late-planted fields may be at risk but crop injury would be later than in areas with earlier flights.
Leaf feeding on emerged corn or weeds will be visible for the next couple of weeks. By early June larvae will be large enough to cut off small corn plants (Table 2).
This has been a relatively low year for black cutworm flights into Minnesota. This does not mean that 2022 crops are without risk of crop injury from black, or other cutworms.
Projections for when black cutworms will be large enough to cut corn moved ahead slightly over the past week. Be advised that temperatures for dates beyond May 25 are projections based on historical air temperature data. Also, remember that moist soil and residue cover will tend to create cooler temperatures where the cutworms spend most of the daylight hours.
Table 1. Positive captures in pheromone traps reported April 30 - May 6, 2022. Numbers in parenthesis indicate multiple captures for that trap and date.
Table 2. Black cutworm development projections based on historical average temperatures. Midwest Regional Climate Center U2U https://mygeohub.org/groups/u2u/purdue_gdd
Based on temperatures from the end of April flight, leaf-feeding should be beginning and small broadleaves may be cut and wilting. Pay particular attention to fields in areas near counties with significant captures and where spring tillage had not occurred by May 1. Where tillage and planting had not occurred by May 19-20, damage from the May 20 flight into SE MN will occur later.
The following provides tips on economic threshold and control for black cutworm.
From Black Cutworm Facts Revised - April 2021
Economic thresholds – When to treat a problem
Cutworms reduce yield by decreasing the final stand or plant population. The generic economic threshold for black cutworm in corn is 2-3% of the plants cut or wilted when the larvae are less than ¾ inch long. The threshold increases to 5% cut plants when larvae are larger. However, with high corn prices, these thresholds could be lowered to 1% wilted or cut for small larvae and 2-3% wilted or cut for large larvae.
Remember to take into consideration corn populations in individual fields and adjust threshold numbers accordingly. For example, if the current plant population is at or near yield-limiting levels, you can afford to lose fewer plants than in a field with a higher emerged population. Table 3 shows the role of corn plant stands in determining yields. Planting date factors into replant decisions as the season progresses. Finding a stand loss late compounds the replant decision. The UMN Extension publication “Corn grower's guide for evaluating crop damage and replant options” includes information on the effect of corn stand, planting date, and hybrid relative maturity on yield.
The reason the black cutworm economic threshold varies by larval size is based in remaining larval feeding. Cutworms must shed their skins (molt) to grow. The stage between molts is called a larval instar. Cutworms will begin to cut corn at the 4th instar (~½ inch long). The smaller larvae tend to cut corn at or near the soil surface while larger larvae tend to feed below ground. The larvae are full grown and cease feeding between 1½ and 2 inches long. While larger larvae will cut or tunnel into larger plants, they have less time left to feed and as a result, cut fewer plants. Table 4 and Figure 23 give approximate sizes in body length and head width for black cutworm larval stages.
There are more detailed dynamic black cutworm thresholds available that use stand, crop stage, projected damage and crop price. However, caution is advised when dynamic thresholds generate lower thresholds below those described above. Yield loss, actual or measurable, does not begin with the first missing corn plant. High grain prices and a good planted and emerged stand means you could easily be treating cutworm populations that would not reduce stand enough to hurt yields.
The rescue insecticide calculator (Table 5) is adapted from a University of Illinois publication and is an example of a dynamic threshold that is used in several management guides. Modern corn yields and prices could indicate treatment at a very low percentage of cut plants using this worksheet, perhaps leading to over-reactive treatment decisions. However, the yield loss factors are still useful when combined with yield loss by stand reduction charts.
Black cutworm Control
Bt Hybrids
Bt hybrids containing the Cry1F protein (Herculex /HX1) or Vip3a protein (Viptera), alone or in stacks, are labeled as controlling black cutworm. While they reduce risk, they might still be damaged under heavy cutworm pressure. An at-plant insecticide is probably not that helpful in preventing additional cutworm stand loss when added on these hybrids. Remember, the Cry34/35 Ab1 (Herculex RW protein) is not the same as the Cry1F above-ground protein. In addition to timing of tillage, spring weed and cover crop growth, the Bt protein in the planted hybrid can help prioritize fields for scouting. The production shows which Bt proteins control black cutworm and several other insect species.
Seed applied insecticides
High rates of neonicotinoid seed treatments (e.g., Poncho, Cruiser, Gaucho) are very effective on many seed and seedling insects and they can provide some protection against black cutworm. They may not always provide satisfactory cutworm control, however. Seed-applied diamide insecticides (e. g. chlorantraniliprole) can also affect black cutworm larvae.
Large numbers of late-instar cutworms moving from weeds to take a bite of corn can overwhelm seed-applied insecticides and Bt in corn tissues
Concerns about Bt-resistant corn rootworm populations have led some farmers and ag advisors to add a soil insecticide to Bt-RW corn for added root protection. That decision, however, should be considered an entirely separate issue from cutworm management.
At-Plant insecticides
Soil-applied insecticides at planting can provide control of cutworm larvae. However, they are not recommended as insurance applications for two reasons. At planting, it is difficult to predict which individual fields will have economically damaging cutworm infestations. Secondly, post-emerge insecticide rescue treatments work very well and can be targeted to fields with actual infestations.
T-band applications for granular insecticides, if so labeled, are sometimes more effective on cutworms than in-furrow applications. However, a T-banded insecticide has its own performance risks and the placement is not necessarily more effective on corn rootworms. Incorporate the insecticide bands as indicated on the label. Windy planting conditions reduce the accuracy of band placement and later blowing of loose, dry soils can also reduce the efficacy of non-incorporated bands. Always read the pesticide labels and use the appropriate rates.
Post-plant rescue insecticides
Fortunately, cutworms are controlled well with rescue insecticide applications, and many post-plant insecticide products provide effective control of black cutworms. Several compounds within the pyrethroid, organophosphate, carbamate, and diamide groups are labeled for post plant/post-emerge cutworm control. Spot treatments can be effective when combined with careful scouting.
Black cutworms tend to remain lower in the soil when the top few inches of the soil profile are dry, meaning that insecticide applications can be less effective. A rotary hoe or row cultivation before application (or after application if below-ground feeding continues) can help improve the efficacy of some insecticides by incorporating insecticides and encouraging cutworm movement. Good coverage of the row area and plants is important. Do not skimp on water and match spray volume and pressure to nozzles designed for insecticide application. Although spraying in the late afternoon or evening will place the insecticides in the field closer to larval activity, it can also reduce control if a temperature inversion prevents the spray from settling on the field.
It is important to read pesticide labels. Be cautious of potential interactions between some organophosphate insecticides (e.g., Thimet, Counter, Lorsban) and some ALS herbicides. These interactions can cause severe or temporary crop injury. Some PPO, and HPPD herbicides may also interact with insecticides.
Make sure cutworms are still present and actively feeding if you decide to treat. Check just before spraying to ensure stand loss is still progressing.
Cultural control
Maintaining good early-season weed control, including the timely termination of cover crops, can reduce the attractiveness of fields to egg-laying females.
Tillage after egg-laying has little impact on either egg or larval survival unless the field is kept black for a couple of weeks after egg hatch. This is long enough to starve the larvae but unfortunately, is a yield-adverse planting strategy.
Until next week,
Bruce Potter and Travis Vollmer